1. Field of the Invention
This invention relates to a high efficiency coding apparatus applied to a picture signal and, more particularly, to a high efficiency coding apparatus in which the transmission rate of recorded data is controlled to correspond to the capacity of a transmission path when a digital video signal is recorded on a magnetic tape.
2. Description of Prior Art
The present applicant has proposed a high efficiency coding apparatus for obtaining a dynamic range equal to the difference between a maximum value and a minimum value of plural picture elements contained in a two-dimensional block and for performing coding adapted to the obtained dynamic range as described in the specification of the Japanese Patent Application No. 266407/1984. Further, as described in the specification of the Japanese Patent Application No. 232789/1985, a high efficiency coding apparatus has been proposed for the coding adapted to a dynamic range with respect to a three-dimensional block formed of picture elements of areas respectively contained in plural frames. Moreover, a variable-length coding method for varying the bit number depending on a dynamic range so as to keep constant the maximum distortion occurring at the time of quantization has been proposed in the specification of the Japanese Patent Application No. 268817/1985.
The above-mentioned high efficiency coding adapted to the dynamic range (called "ADRC") enables great compression of the amount of data to be transmitted and is therefore suitable for application to a digital VTR. Particularly, the variable-length ADRC can enhance the compression rate. However, since in the variable-length ADRC, the data amount to be transmitted varies depending on the contents of pictures, buffering processing is necessary when employing a transmission path with a fixed rate, such as a digital VTR for recording a predetermined amount of data as one track.
For a buffering system of the variable-length ADRC, as described in the Japanese Patent Application No. 257586/1986, the present applicant has proposed the use of a buffering system in which distribution of cumulative-type dynamic ranges is formed, a set of threshold values prepared in advance are applied to such distribution, the amount of data generated in a predetermined period, for instance, in one frame period is obtained, and the generated data mount is controlled so as not to exceed a target value.
FIG. 10 shows a graph of cumulative-type distribution referred to in such Japanese patent application No. 257586/1986. In FIG. 10, the abscissa is the dynamic range, and the ordinate is a number of occurrence. T1 to T4 depicted in the abscissa are threshold values. The bit number of quantization is determined by the threshold values T1 to T4. In the case of the dynamic range DR within the range from a maximum value to T1, the bit number of quantization is made to be 4; in the range of T1-1 to T2, the bit number of the quantization is made to be 3; in the range of T2-1 to T3, the bit number of quantization is made to be 2; in the range of T3-1 to T4, the bit number of quantization is made to be 1; and in the range of T4-1 to a minimum value, the bit number of quantization is made to be zero (that is, no code signal is transmitted).
With respect to the cumulative-type distribution, the occurrence numbers ranging from the threshold value (T1-1) to the threshold value T2 are accumulated with regard to the occurrence numbers of dynamic ranges from the maximum value to the threshold value T1 in order to obtain distribution of the dynamic range DR in a frame period. The occurrence numbers from the threshold value (T2-1) to the threshold value T3 are accumulated similarly. Similar processing is repeated thereafter. As a result, the occurrence number at which the dynamic range DR is the minimum value is equal to the total number (M.times.N) of the block contained on one frame.
In this manner, when cumulative-type distribution is formed, the cumulative number up to the threshold value T1 becomes x.sub.1, the cumulative number up to the threshold value T2 becomes (x.sub.1 +x.sub.2), the cumulative number up to the threshold value T3 becomes (x.sub.1 +x.sub.2 +x.sub.3), and the cumulative number up to the threshold value T4 becomes (x.sub.1 +x.sub.2 +x.sub.3 +x.sub.4). As a result, the amount of generated information (total bit number) for a period of one frame is shown by the following equation: ##EQU1##
The threshold values T1 to T4 are selected so that the above-mentioned amount of generated information does not exceed a target value. When an optimum value is obtained with the alteration of the threshold values, said values x.sub.1 to x.sub.4 are changed depending on the threshold values, and the generated information amount is calculated for every set of each threshold value. Therefore, once a cumulative-type distribution table is formed, the calculation of generated information can be effected easily.
A system for converging the rate of transmission data into a target value by changing the four threshold values in the level direction, for instance, as mentioned above, is not sufficient in performance, for example, in reducing distortion of quantization noises, etc.